An advantage of cold-ventilation refrigerators/freezers is the absence of frost in the refrigerating compartments. Nevertheless, frost is formed on the coldest spot of the freezer, i.e. on the evaporator fins. If freezer or refrigerator doors are opened frequently so much frost will be formed on the evaporator that it obstructs the fins, thereby reducing the refrigeration efficiency and impeding the circulation of air. Therefore, the evaporator must be defrosted periodically.
It is customary to effect defrosting by means of a resistive heating element which is in contact with the evaporator. By means of a timer this element is energized at regular intervals (generally for half an hour every 8 hours). In order to avoid an excessive temperature rise in the freezing compartment said element should not be energized for too long a time and should be disconnected as soon as defrosting is completed.
The problem which then arises is that the instant at which defrosting is finished should be detected in order to stop the defrosting process.
A solution which may be adopted is to detect the flow of water in the discharge pipe, for example by measuring the electrical resistance between two electrodes as described in the document FR 2,609,789. However, these measurements are susceptible to errors and are not reliable enough to enable the end of the defrosting operation to be determined correctly.